The present invention relates to a hip prosthesis provided with spherical head, neck and stem with straight side, which allows the nailing of the medullary cavity of the already prosthetized, then fractured femur.
The number of those living with hip prosthesis is increasing more and more at the present time. The number of implanted artificial hip joints is estimated to be two thousand per day. The further increase of the cases into the millions is accounted for by the fact that the diseases of the hip joints due to wear are increasing numerically along with the rapid rise of the age of the population accompanied by a corresponding development of the medical technique.
Recently, however, some disadvantageous properties of hip prosthesis came to light, which initially were not noticeable. It was found that the hip prosthesis is prone to cause fracturing of the femur. This is proven by the increasing number of fractures occurring after prosthesis implantation, and the theoretical explanation of this phenomenon is as follows: the metals implanted into the organism and adapted to the bone do not have the identical elasticity as that of bone. Be it either a plate used for healing of a bone fracture, screwed to any tubular bone, or a metal cemented into the medulla, eventually a fatigue phenomena appears along the boundary of the two types of elastic material, which leads to fracturing of the bone.
The regenerating, defensive mechanism of the living bone working to prevent a fatigue fracture is inhibited particularly in this case--by the fact, that the blood supply of the bone is reduced: about 30% of the blood supply proceeds from the periosteum, and 70% from the medulla. The medulla /replaced by the prosthesis/ is removed, while the bone-glue used for cementing the prosthesis results in further bone damage through the exothermal evolution of heat during the polymeric bond of the bone-glue.
Prosthesis are frequently replaced as a result of dislocation, wear or for other reasons. Replacements are made possible in many cases only by one technique, the surgeon prepares a "knock-out window" at the end of the prosthesis in the bone--the vicinity most prone to fatigue--which extends through one quarter of the circumference of the tubular bone. Thereby breaking down its statics, and damaging the periosteum, which now alone supports the bone. This is proceeded by mechanical and thermal damages. The bone window is refastened generally with additional metals which produce anodic-cathodic effects in the living organism, due to their different metallurgical compositions. This so-called metallosis further damages the already worn out upper third part of the femur, where the stem of the prosthesis ends.
If this part of the non-prosthetized, healthy femur fractures--e.g. in case of accident--the following surgical interventions are possible:
Nailing of the medullary cavity with Kuntscher nails. These are shorter or longer shaped steel tubes split along the surface, the lower end of which is provided with edge. These, hammered into the medullarly cavity along the fractured parts, provide stable anchorage, whereby the injured person regains his mobility quickly. Accordingly, this method is used most frequently.
Another method is anchorage with plate and screws, but its stability never reaches that of the nailing of the medullary cavity. At the same time it requires major surgical exposure, repeated operation /removal/, and it causes muscle damage and considerable stress. Breakage of the plate, and repeated fracture of the bone frequently occur. For this reason it is used only in exceptional cases.
Further possibility is the external bone anchorage of the damaged femur, but this is executed in exceptional cases as an emergency method in case of certain open, complicated bone injuries.
The conventional plastering is also known, this however rarely ensures adequate positional bone healing. It is particularly unfavourable in fractures of the femur, because the time of healing is delayed, thereby multiplying the known shortcomings of these treatments generally in case of elderly patients.